1. Field of the Invention
The present invention relates to a method for producing an ethylene-vinyl alcohol copolymer (hereinafter abbreviated as “EVOH”) resin composition.
2. Description of the Related Art
EVOH has a high gas barrier property and excellent oil resistance, aroma retention property, transparency, and the like. Due to these properties, EVOH is formed into films, sheets, bottles etc. and widely used for food packaging and the like.
Conventionally, various properties of EVOH, such as flexibility, impact resistance and moldability, have been improved by blending the EVOH with various resins. Addition of a starch-based polymer to EVOH is described in JP05 (1993)-9334A. In JP05 (1993)-9334A, EVOH pellets having a water content ranging from 20 wt % to 60 wt % and raw starch (corn starch with a water content of 12 wt %) are introduced into a Henschel mixer and mixed. The mixture is fed to a twin-screw extruder with a vent-port so as to be kneaded at a temperature of 120° C., melt-extruded in a strand form and pelletized. However, the starch-based polymer is not dispersed in the EVOH with sufficient homogeneity by this method. As a result, high quality molded products are not obtained due to starch coagulation in the molded products.
JP05 (1993)-93092 A describes a method for producing a resin composition comprising starch and EVOH. In JP05 (1993)-93092A, a mixture of starch and glycerol is fed to a twin-screw extruder and melted at 170° C. while EVOH is fed from a side feeding port of the twin-screw extruder, so that the mixture and the EVOH are melt-kneaded in the twin-screw extruder. In this method, the EVOH is previously melted by heating at 210° C. in a single-screw extruder. However, since the temperatures in the respective extruders are set as high as 170° C. and 210° C., the EVOH thermally deteriorates in the extruders, and this often causes coloring of the final resin composition. Moreover, since foaming due to decomposition gas occurs at high temperature, pellets with good visual properties are difficult to obtain.
It is also known that various properties of EVOH can be improved by adding inorganic materials. For example, JP05(1993)-39392 A describes the addition of water-swelling phyllosilicate for improving the gas barrier property of EVOH. Specifically, JP05(1993)-39392A proposes a method of adding EVOH and methanol to an aqueous dispersion of the silicate (e.g., montmorillonite) so as to prepare an EVOH solution. The solution is cool-solidified, pulverized, and dried. However, volatilization of the methanol causes problems of working environment.
In another example described in JP2000-43038 A, an oxide such as silicon oxide is added to EVOH in order to improve lubricity for melt-molding the EVOH. JP2000-43038 A discloses a method for producing an EVOH resin composition, in which an EVOH solution is precipitated in a coagulation bath after adding fine particles of oxides. Similarly, volatilization of alcohol absorbed in the precipitation step causes problems of working environment.
JP10(1998)-158412 A also proposes the addition of clay minerals such as montmorillonite to a vinyl alcohol copolymer such as EVOH. Specifically, JP10(1998)-158412 A describes a method of melting and kneading in an extruder a vinyl alcohol copolymer including a dry mineral, and further adding water to the copolymer. Since the vinyl alcohol copolymer requires melt-kneading at a high temperature in this method (EVOH is heated to 220° C. in the Examples of the application), coloring due to thermal deterioration is a problem, especially when EVOH is used.
JP2000-191874A proposes a method of melt-kneading in an extruder a water-swelling layered inorganic compound like montmorillonite together with EVOH having a water content ranging from 25 wt % to 50 wt %. In this method, likewise, the inorganic compound is added as a powder. Since water-containing EVOH is used, the temperature for melt-kneading can be set lower in this method. However, it is not easy to obtain sufficient dispersion and cleavage of the inorganic compound simply by blending the powder in the extruder and kneading mechanically. The inorganic compound needs to be swollen with water contained in the EVOH while an excessively high water content will hinder molding of the EVOH, and thus, the water content of the EVOH should be controlled to within a relatively narrow range.